Numerous research studies have been carried out on improving the performance of primary lithium batteries.
Some of these research studies have been directed at the composition of the electrodes of such batteries, in particular the cathode.
Thus, primary lithium batteries comprising a manganese oxide cathode have an energy density of 150 to 330 Wh·kg−1, lithium batteries with an electrode making it possible to release SO2 have an energy density of 150 to 315 Wh·kg−1 and lithium batteries having an SOCl2 electrode have an energy density of 220 to 560 Wh·kg−1.
Finally, lithium batteries with an electrode made of fluorinated carbon of formula CFx, with x representing the F/C molar ratio varying between 0.5 and 1.2, have an energy density of 260 to 780 Wh·kg−1.
The fluorinated carbons having the composition CF1 can deliver a theoretical capacity of 865 mAh·g−1 when they are used as primary lithium battery electrode material. The increase in content of fluorine above CF1 (CF1.2) is not beneficial for the capacity due to the creation of electrochemically inactive CF2 and CF3 groups.
This theoretical capacity corresponds to the electrochemical conversion of every C—F bond.
This is because, within the primary lithium battery, the electrochemical process in a fluorinated carbon (CFx) electrode involves the cleavage of the C—F bond by contributing an electron from the external circuit. The fluoride ion then formed combines with a lithium ion originating from the electrolyte to form LiF.xLi→xLi++xe−CFx+xLi→C+xLiF
This reaction is irreversible. In order to obtain the maximum capacity (or amount of current, for the battery), the strategy has thus for a long time consisted in choosing a fluorinated carbon exhibiting the highest possible degree of fluorination, that is to say a CF1 composition (each carbon atom is bonded to a fluorine), indeed even CF1.1-1.2 composition (for compounds which are weakly organized structurally, such as petroleum cokes with small sizes of graphite sheets, CF2 and CF3 groups may be formed during the fluorination). This strategy exhibits a major disadvantage, the insulating nature of the highly fluorinated CFx, which generates excess voltages in the battery and lowers the Faraday efficiency (the ratio of the experimental capacity to the theoretical capacity).
Furthermore, Yasser Ahmad et al. have described, in “The synthesis of multilayer graphene materials by the fluorination of carbon nanodiscs/nanocones”, Carbon, 50 (2012), 3897-3908, subfluorinated carbon multisheet nanomaterials obtained by the “subfluorination” process.
This process is characterized by two essential points: the starting material is a nanomaterial and the fluorination is a subfluorination (a portion of the carbon atoms remains nonfluorinated) obtained either by direct fluorination with molecular fluorine (F2) or by controlled fluorination using a solid fluorinating agent TbF4.